When a three-phase brushless DC motor is controlled by an inverter control, upper-arm switching elements and lower-arm switching elements of a three-phase inverter circuit are switched around so as to convert a DC power to a three-phase AC power.
Methods known as inverter conduction control methods include a 120-degree conduction method in which the ON period of each switching element is set to an electrical angle of 2π/3, and a 180-degree conduction method in which the ON period of each switching element is set to an electrical angle of π. In order to control the torque of the three-phase brushless DC motor, the switching elements of the three-phase inverter circuit are controlled by a pulse width modulation (PWM) control.
With the aforementioned three-phase brushless DC motor control method, when an upper-arm switching element (or a lower-arm switching element) of a predetermined phase is blocked for a PWM control, a magnetic energy that has accumulated in the inductance of the three-phase brushless DC motor is urged to continuously flow during the OFF period, due to the large inductance of the three-phase brushless DC motor.
In order to address this issue, a recirculation diode (flywheel diode) is connected in a reverse parallel position with respect to each switching element to demagnetize the magnetic energy, which has accumulated in the inductance, through the flywheel diode.
In a PWM control, a high voltage in the reverse direction is applied immediately after a current in the forward direction passes through a flywheel diode. At this point, the flywheel diode instantaneously passes a current flowing in the reverse direction, called a “recovery current”. A recovery current is a power that is unnecessary for driving a motor, and is consumed as heat by an inverter circuit, thereby being a cause for a decrease in the power conversion efficiency of the inverter.
Where a Metal-Oxide-Semiconductor (MOS) transistor is used as a switching element, the parasitic diode of the MOS transistor is used as the flywheel diode. However, the parasitic diode of a MOS transistor has a long recovery period, during which a recovery current flows. Therefore, due to the recovery current, there is a substantial power loss, and heating is likely to occur.
Where an IGBT (insulated gate bipolar transistor) is used as a switching element, since it has no parasitic diode, it is necessary to provide an external flywheel diode. By using a fast recovery diode (FRD) with less recovery current as an external flywheel diode, it is possible to reduce the switching loss (see, for example, Patent Document 1).    Patent Document 1: Japanese Published Patent Application No. H07-222459